I need to , while i am working offline. I know there is  a vba code to do this but can't find any on the internet.

The goal here is to generate mail merge with attachment. While all mail merged will be in outbox as I am working offline, I will attach a file to  it then turn work online to send the mail. If there is a simpler way then you are welcomed but no third party add ins or software as company policy do not allow it.

You can use a macro to add the attachment automatically on send or as you create the merge. 

I have a macro that adds the attachment (and sets other fields)  as the message is sent here: http://www.slipstick.com/developer/code-samples/mark-mail-merge-messages-as-urgent/ 

links ot other macros and 3rd party utilities are here: http://www.slipstick.com/outlook/mail-merge-using-outlook